Identification of appropriate CFD models for simulating aerosol particle and droplet indoor transport

Computational fluid dynamics (CFD) has been widely used to predict indoor particle and droplet transport and dispersion. CFD solves simplified conservative equations that describe the major characteristics of particle and droplet indoor movement, along with the flow governing equations. This paper reviews the principles of three prevalent CFD models for indoor particle and droplet simulation: the lazy particle model, isothermal particle model and vaporizing droplet model, with a focus on the disparities between these models. The study verifies that different particle and droplet models provide distinct simulation results in which size of particle and droplet is a critical factor. To justify proper application of these models for particles and droplets with different sizes, the paper theoretically analyzes the Lagrangian transport equations for particle and droplet and identifies two crucial time numbers particle momentum response time and evaporation lifetime. Upon these numbers, two new indices have been introduced - Stokes number and evaporation effectiveness number, which can be used as simple criteria to guide the model selection. The case studies confirm the value of the indices and provide the rules of thumb for determining appropriate CFD models for particle and droplet indoor transport under typical room conditions.